Blender Game - How do I start the game with the cursor centered?
According to the description you really want to let the ship fire projectiles toward a point under the system mouse cursor. 

I suggest to use a short Python script to position an "aiming target" empty at the desired location (e.g a collision point under the cursor). Then let the turrets turn towards that aiming target (TrackToActuator).

Setting the system mouse cursor to the middle of the screen at game start would still add to the player's comfort.